1. Field of the Invention
The invention relates to electrical circuit breakers having an electronic processing circuit for processing one or more sensor signals and providing a trip signal to open the controlled circuit. Most circuit breakers in use today have a mechanically operated switch or contact set; when electronically tripped, typically the trip signal is a current pulse through a solenoid which unlatches a spring-loaded contact set. The invention is equally applicable to a "solid state circuit breaker" which changes the state of an element from highly conducting to non-conducting.
2. Description of the Prior Art
U.S. Pat. No. 4,589,052 describes an electronic processing unit for an electronically tripped circuit breaker, having separate rating settings for instantaneous trip, and short time and long time overloads and GFI settings, using adjustable resistor values. The overall rating is set by a resistor, while trip times for instantaneous trip, short and long time overload are selectable by a switch.
This patent describes the use of digital circuitry for a current squaring function, overcurrent pickup functions, and time delay function. In a 3.phi. breaker, 4 current transformers are used to provide a composite current value, a ground fault current signal, and power for the electronic circuitry. Adjustable settings establish the level of current which will be carried indefinitely without tripping, and the overcurrent pickup (threshold) values for computing a sum of current squared multiplied by time in response to a long time relatively small overcurrent, for computing a sum of current squared multiplied by time in response to a short time relatively large overcurrent, and for instantaneous tripping in response to a very large overcurrent.
A so-called peak detecting A/D circuit provides a digital output, starting from zero once every 20 milliseconds, which rises to a value corresponding to the peak value of the composite signal during that 20 ms. This digital output is used to determine if the current exceeds any of the pickup values, and also is provided to an "i.sup.2 t " circuit whose output is a frequency f.sub.p which is proportional to the square of the A/D digital output.
The long time pickup line 50 provides a binary signal indicating whether or not the peak-detected current exceeds the long time pickup level. If this level is exceeded, then a long time counter 115 counts up at a frequency f.sub.p /nm; while, if the level is not exceeded, the counter 115 counts down at a different predetermined frequency f.sub.cool LT to provide a "cooling" effect. If the counter 115 output exceeds a predetermined value, a trip signal is produced. As shown in FIG. 5, a similar arrangement is provided, to operate with different pickup and timing values, to generate a trip signal if for a shorter period of time the peak detected current exceeds the short time current pickup limit.
As the above description points out, the breaker has fully independent timing circuitry for current exceeding the short time pickup level, and current exceeding the long time pickup level. As a result, a fairly large overcurrent which exceeds the short time pickup value, and lasts until the relevant counter has reached 99% of the trip value, and then drops to a value just less than the short time pickup value, will not cause tripping via the short time circuitry; however, the long time counter 115 operates at a different scaling factor, and will be at substantially less than 99% of its trip value when the drop occurs. Thus the independence of the circuits reduces the degree of protection.
Further, no non-volatile semiconductor elements are used for storing pickup or overcurrent time values; and the power supply for the electronics package is energized through the current transformers. If the circuit breaker is turned off for a second or so, or load current in all the load conductors falls to zero and remains at that value long enough so that energy stored in any filter capacitor in the power supply is largely exhausted, the values stored in the counter 115 and short time counter will be lost. As a result, if the overcurrent condition is resumed before the load wiring or devices have been deenergized long enough to cool down or otherwise recover from the previous overcurrent, dangerous overheating or other damage may occur before the breaker trips.
Another area of concern has been determination of the RMS value of the current being measured. Thermally operated circuit breakers inherently were sensitive to the RMS value, or heating value, of the current being measured. If, however, digital sensing is desired, then the sampling technique can make matching of a desired trip time delay curve unreliable.
U.S. Pat. No. 4,741,002 discloses a circuit for calculating the RMS value of a digitally sampled current. A voltage to frequency converter has an output frequency proportional to the instantaneous current value. This output is fed to a frequency squaring circuit, whose output frequency is then proportional to the square of the instantaneous current. The pulses of this output frequency are then accumulated for a period of time, and the square root of this accumulated value is calculated. This circuit has the disadvantage that, if the averaging period for calculation of the RMS value is very long, then the numbers become large and a high calculating rate and capacity are required.